Microbial Colonization of Space Stations
Long-term space station missions to date include Salyut, Almaz, Skylab, Mir, ISS (International Space Station), and Tiangong, with the latter two still ongoing. These expeditions demonstrate that space stations support a diverse microbial community. While most microbes are not harmful, routine microbial monitoring onboard space stations is still essential in preventing material biodeterioration and maintaining crew health. As currently identified, these microorganisms originate from Earth, with main source coming from crewmembers themselves and from materials manufactured on ground.  Environmental monitoring is important in identifying microbe and providing data for future studies in attempt to minimize microbial threat to crewmembers’ performance and spacecrafts’ health.
Microorganisms exist in the portable water, ventilating air, and on surfaces of space stations.  While these microorganisms share the same living space as the crewmembers, the crewmembers themselves also serve as incubators for various microbes.  Researchers have identified several strains of bacteria and fungi as opportunistic pathogen that may pose threats to crewmember. (refer to Key Microorganism section) Moreover, they have also identified the type of microbes responsible for biodegradation of structural materials.  (refer to Biodegradation section)
Microbial Diversity and Interactions
Other than in portable water and air, bacteria and fungi typically co-exist in biofilms on surfaces, including water pipe lines.  Biofilms provides suitable growth conditions, and intrinsically contribute to antibiotic and biocide resistance due to horizontal gene transfer of antibiotic resistance plasmid, and the fact that microbes positioned inside biofilms are harder to reach. One study shows that as much as 36 species of fungi and 58 species of bacteria can co-exist in a single biofilm. However, the composition varies drastically due to dynamic (changing) environmental conditions, available nutrient (surface materials), and also the thickness of biofilm. In addition, exchange of microflora carried by crewmembers travelling to and from the space craft also increases microbial diversity onboard space stations.
Space Microbial Processes
Biodegradation of the surfaces of spacecraft structural materials are results of colonization of bacteria and fungi.  While fungi are more abundant in early colonization and biofilm formation on polymeric materials, given sufficient humidity, subsequent microbes thrive in biofilms that forms at wide ranges of temperatures. Biodegradation of bacteria and fungi predominantly occur on these surfaces due to the availability of carbon in surface finishings, as well as other nutrients present in structural materials (polymeric-base and metal-based). In thin biofilms, oxic condition support aerobic biodegraders; in thicker biofilms, where oxygen diffusion is limited, anaerobic fermenters and metal reducers are come into play. Although these biodegradation of carbon (polymeric)-based and metal-based materials are due to similar hydrolysis reactions, the yielding products and degree of degradation are different depending on different microbes. 
According to various in vitro studies, the microgravitaional environment in space enhances microbial growth.  Several factors were examined in a closed, isolated environment on ground-based labs, and the results show a decreased lag phase and increased final yield compared to normal-gravity conditions.  This enhanced microbial growth also contribute to increase of antibiotic resistance.  In addition to the confined, microgravitaional condition, various unknown factors onboard also provide distinctive selective pressures that favor certain microbes that eventually become harder to eliminate.
Microbe Identification Techniques (Onboard Environmental Monitering)
Most antimicrobial and environmental monitoring implementations on ISS are results of studies done by previous expedition, Mir and Skylab. Before December 2006, isolation of microorganisms depended on the growth of environmental isolates on agar media onboard space stations and the subsequent ground-based analysis back on Earth.  However, this method restricts the number identifiable microorganisms since more than 95% of environmental microbes do not grow on conventional agar growth media. Furthermore, accuracy is limited since months often passed before returning to Earth for further analysis.  More recently, ISS launched Lab-On-A-Chip Application Development Portable Test System (LOCAD-PTS).  This new technology can be hand-held and allowed rapid microbial monitoring of bacteria and fungi since its implementation in December 2006. 
Dominant bacteria are Acidovorax spp., Afipia spp., Brevundimonas spp., Propionibacterium spp., Serratia spp.,  Sphingomonas spp., and Methylobacterium spp.,  and opportunistic human pathogens include Afipia spp., Brevundimonas spp., Propionibacterium spp., Serratia spp., and Sphingomonas spp. (rarely pathogenic to human).
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